7.  But let us return to the examination of the ingenious contrivances of creation.  How many trees then arose, some to give us their fruits, others to roof our houses, others to build our ships, others to feed our fires!  What a variety in the disposition of their several parts!  And yet, how difficult is it to find the distinctive property of each of them, and to grasp the difference which separates them from other species.  Some strike deep roots, others do not; some shoot straight up and have only one stem, others appear to love the earth and, from their root upwards, divide into several shoots.  Those whose long branches stretch up afar into the air, have also deep roots which spread within a large circumference, a true foundation placed by nature to support the weight of the tree.  What variety there is in bark!  Some plants have smooth bark, others rough, some have only one layer, others several.  What a marvellous thing!  You may find in the youth and age of plants resemblances to those of man.  Young and vigorous, their bark is distended; when they grow old, it is rough and wrinkled.  Cut one, it sends forth new buds; the other remains henceforward sterile and as if struck with a mortal wound.  But further, it has been observed that pines, cut down, or even submitted to the action of fire, are changed into a forest of oaks. The phenomenon has been observed in later days, though Basil may be at fault in his account of the cause.  When pines have been cleared away in North American forests young oaklings have sprung up.  The acorn lay long hid, unable to contend against the pine, but, when once the ground was clear, it sprouted.  This upgrowth of a new kind of tree had been accounted for partly by the burial of germs by jays, rooks, and some quadrupeds; partly by the theory of De Candolle and Liebig that roots expel certain substances which, though unfavourable to the vitality of the plant excreting them, are capable of supporting others.  So, on the pine pressure being removed, the hidden seeds sprout in a kind of vegetable manure.  cf. Sir Charles Lvell’s Travels in the United States and Rough’s Elements of Forestry, p. 19.  We know besides that the industry of agriculturists remedies the natural defects of certain trees.  Thus the sharp pomegranate and bitter almonds, if the trunk of the tree is pierced near the root to introduce into the middle of the pith a fat plug of pine, lose the acidity of their juice, and become delicious fruits. Ambrose, Hexæm. iii. 13, writes:  Amygdalis quoque hoc genere medicari feruntur agricolæ, ut ex amaris dulces fiant fructus, ut et terebrent ejus radicem arboris, et in medium inserant surculum ejus arboris quam Græci πεύχην, nos piceam dicimus:  quo facto succi amaritudo deponitur.  Let not the sinner then despair of himself, when he thinks, if agriculture can change the juices of plants, the efforts of the soul to arrive at virtue, can certainly triumph over all infirmities.